LM74700-Q1: Higher than 75V protection scheme

Rene Straub

Part Number: LM74700-Q1

Hello TI,

we plan to implement a reverse polarity protection based on LM74700-Q1. However our worst case voltage anode - cathode is > 75V. The design idea is to have a Zener/TVS across cathode/anode).

The TVS would support the FET in forward direction when the FET is off (parallel to body diode). When anode goes below cathode by the TVS voltage (say 60V), the TVS would clamp and protect FET and LM74700-Q1. Some amount of current would be pulled back via TVS to source.

Any opinions on that solution from TI side?

Best regards

-Rene

over 6 years ago

0 Karikalan Selvaraj over 6 years ago

TI__Genius 16305 points

Hello Rene,

Thanks for posting the question, let me check on this and get back sooner tomorrow.

Regards,
Kari.

0 Karikalan Selvaraj over 6 years ago in reply to Karikalan Selvaraj

TI__Genius 16305 points

Hello Rene,

Yes adding a TVS in the forward path would help protect LM74700 and the MOSFET (Vds) during reverse condition at the input.
However, if the anode is pulled down by a stronger power source, cathode will get dischaged just enough to meet the TVS voltage clamp/current.

What is the maximum input voltage expected?

Regards,
Kari.

0 Rene Straub over 6 years ago in reply to Karikalan Selvaraj

Prodigy 100 points

Hello Kari,

We simulated the circuitry and noticed this exact behavior. A test pcb is currently being assembled and will be tested next week.

The max. Operations voltage is 36v. During iso 7637 Tests it will be 28v. Thus a smdj33c TVS has to be used as first stage protection. Max. Clampinging voltage is around 53 volts.

Best regards

René

0 Karikalan Selvaraj over 6 years ago in reply to Rene Straub

TI__Genius 16305 points

Hello Rene,

Is this being used for a 12V battery application or 24V application?
For a 12V battery, max voltage is about 16V.

During the ISO test with max voltage of 16V at output, if the ISO pulse 1 is applied it should be safe to use SMBJ33CA.
75V - 16V = 59V and this is greater than the max clamping voltage of SMBJ33CA.

Do you expect a ISO pulse 1 after a load dump for 28V?

Regards,
Kari.

0 Karikalan Selvaraj over 6 years ago in reply to Karikalan Selvaraj

TI__Genius 16305 points

Hello Rene,

Let us know if you were able to test it on the PCB.
Meanwhile, i would like to close this thread.

Please feel free to reopen this thread when you have the results.

Regards,
Kari.

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LM74700-Q1: Higher than 75V protection scheme